Designing Databases for Historical Research

D. Relationships

D1. Introduction

As mentioned in
Section C, most databases consist of data held in more than
one table, and this is especially true for databases where the
data is derived from historical sources. Relationships are
created between the tables to connect the data in one to the data
in the other: more precisely, relationships are used to connect
specific records in one table to specific records in another. In
many ways relationships, and the whole relational data model,
comprise the most difficult aspect of designing a database, and
not necessarily because they are difficult to actually create.
What is difficult about relationships is why we need them – the
reasons for using related data can seem obscure and unnecessary
at the start of a database project, especially if you have
limited experience of using databases. They are, however,
extremely important. In essence what relationships allow us to do
is two-fold: firstly they allow us to simplify very significantly
the process of data entry (and incidentally at the same time
enable us to protect the quality of the data we enter by limiting
data entry errors); and secondly they serve to ensure that the
results of our queries are accurate by making it clear precisely
what it is that is being queried.